1. Field of the Invention
The present invention relates to a solid state imaging device having a high quality picture and low power consumption, and a method of driving the same.
2. Description of the Related Art
Solid state imaging devices carried in cellular phones or the like include a CCD (charge coupled device) type image sensor and a CMOS type image sensor. The CCD type image sensor is excellent in picture quality and the CMOS type image sensor consumes lower power and its process cost is low. In recent years, MOS type solid state imaging devices using a threshold voltage modulation method which combines both a high quality picture and low power consumption have been proposed. A MOS type solid state imaging device using the threshold voltage modulation method is disclosed in Japanese Unexamined Patent Publication No. 2002-134729, for example.
In the solid state imaging device of Japanese Unexamined Patent Publication No. 2002-134729, the picture output is obtained by arranging unit pixels in a matrix form and repeating three states of initialization, accumulation, and reading. Moreover, in the solid state imaging device of Japanese Unexamined Patent Publication No. 2002-134729, each unit pixel has a photo-diode, a modulation transistor, and an overflow drain gate. The gate of the modulation transistor is formed in a ring shape.
Electric charges (photo-generated electric charges) generated by light incident upon the photo-diode are transferred to a P-type well region formed under a ring gate, and accumulated in a carrier pocket formed in this region. The threshold voltage of the modulation transistor changes corresponding to the photo-generated electric charges accumulated in the carrier pocket. Accordingly, a signal (pixel signal) corresponding to incident light is obtained from a terminal coupled to the source region of the modulation transistor.
In the above described conventional solid state imaging device, a picture signal corresponding to light incident upon the photo-diode in an accumulating period is outputted in a reading period. By accumulating photo-generated electric charges generated by the photo-diode in a carrier pocket, the picture signal corresponding to incidence light is obtained. The photo-generated electric charges accumulated in the carrier pocket are discharged at the time of initialization.
For example, in the device of Japanese Unexamined Patent Publication No. 2002-134729, with respect to a selected line selected line), the photo-generated electric charges of the carrier pocket are read, and with respect to a non-selected line, the photo-generated electric charges remain accumulated in the carrier pocket. An accumulating period, a reading period or an initialization period are set for a selected line and a non-selected line, respectively.
In the device of Japanese Unexamined Patent Publication No. 2002-134729, a flow of the photo-generated electric charges in the accumulating period, reading period, and initialization period is controlled by the potential gradient of each part.
However, the design of a layer thickness and the design of an impurity profile or the like are complicated, and it is extremely difficult to obtain an ideal potential gradient. For this reason, actually, carrier leakage, which the photo-generated electric charges accumulated in the carrier pocket flow out, occurs even in the non-selected lines.
In this case, if the carrier leakage occurs similarly with respect to all unit pixels, there will be no specific problems from a viewpoint of pixel variation, however, in the case where there exists a line in which carrier leakage occurs and a line in which the carrier leakage does not occur, there will be a problem that the dynamic range and the saturation output vary for each of these lines, and thereby the picture quality will degrade.
The present invention has been made in view of this problem, and is intended to provide a solid state imaging device which can make the dynamic range and the saturation output uniform and improve the picture quality by conforming the carrier leakage of all the pixels, and a method of driving the same.